Resistance welding is used widely to join the metal members; for joining steel sheets and the like the process often is automated. Typically, a pair of steel sheets are overlapped, opposed electrodes are brought in from either side to exert substantial pressure on the joint-to-be, a very high current is passed between the electrodes, and a "nugget" is produced that joins the sheets.
The tip of such electrode, which contacts the metal to be joined usually fails because of excessive "mushrooming", a spreading, often splitting, deformation. It is customary to raise welding amperage periodically as such deformation increases with use, this in an attempt to keep the current density desirably high for the formation of suitable nuggets. Eventually, the accumulation of mushrooming and wear no longer can be tolerated and the electrode must be replaced, or, in some cases, redressed. For economy and ease of replacement, electrodes having disposable, replaceable caps today are the most generally used electrode types for production operations.
Electrode force (i.e., pressure exerted by the electrodes on the area to be welded), weld current, and weld time measured in cycles, determine weld size and strength. Compared to welding bare steel sheet, the contact resistance for joining galvanized steel sheet is considerably lower. This is due to the softness of zinc, its fairly low melting point, and its high electrical conductivity. Also, mushrooming zinc tends to ring the weld and shunt current outside the area of the weld. To compensate for this wasted current, spot welds in galvanized steel sheet require higher amperages and longer times than spot welds in uncoated sheets of like gauge. Additionally, the caps fail much more rapidly when used to weld galvanized sheet than bare steel sheet because zinc combines with copper of the caps, forming brass. That has higher electrical resistivity than the copper or original special copper alloy from which the cap was formed. As heat softens the tip, its face starts to mushroom and enlarge. Thus, the resistance welding of zinc-coated sheet, and particularly of double-sided, zinc-coated sheet, is an especially severe service for the caps. Nevertheless, such welding is increasingly sought in the assembly of rust-resisting articles, particularly automobiles.
The manufacture of automobiles is the largest application of resistance welding today. Until fairly recently virtually all the caps generally used in such service were of high strength, high conductivity copper alloys such as copper alloyed with a little chromium, zirconium, cadmium, cobalt, nickel, beryllium, tungsten, molybdenum, or a combination of these alloying agents. Representative alloys of this sort can contain about 0.1% to about 0.2% zirconium and about 0.6% to about 1% chromium. (For brevity, such metals and alloys may be referred to from time to time in this specification simply as "copper alloys", because a copper-base alloy has been for so long the mainstay cap material for resistance welding operations in automobile production).
The shapes of the tips of caps today widely offered commercially are generally four to six in number; apart from their tips their main outlines usually are cylindrical with the maximum cylindrical diameter being between about 12 mm and about 19 mm. Such cap is cooled by a stream of water flowing into and out of an inner recess of the cap from a "shank", i.e., an adapter, that delivers such coolant and conducts it away as well as bringing the electrical power to the cap. For simplicity, the welding tip of a cap here can be termed to be at the "front" of the main body portion of and the connector to such shank at the "rear" of the main body portion of a cap. While a variety of connection types have been proposed for fastening cap to shank, i.e., a threaded joint, etc., the type most generally used now is a locking taper joint (i.e., an RWMA--Resistance Welding Manufacturers' Association--number such as 4, 5, or 6). The cap can have a female or a male taper for such connections, the former probably being the more popular.
Six commercial welding cap end shapes are represented in many suppliers' brochures, and they can be characterized as follows: Pointed--having a frustoconical front end of appreciable taper that usually starts at or near the periphery of the broadest cylindrical section of the cap and ends in a small face; Dome--a rounded face of fairly high arch that extends across the end of such cylindrical section; Flat--a flat welding face which is of the same diameter as such cylindrical section; Offset--such cylindrical section is beveled upwardly from one side to leave a small lunar face on the front end; Cone--a blunt, shallow frustoconical tip projects from such cylindrical section; and Radius--which is a slightly convex bulge clear across the end of such cylindrical section.
There have been relatively few, if any, widely adopted departures shapes, and it would appear that such departures have not been adopted but rather are shunned by the American automobile manufacturing industry. One such "nonconforming" cap was brought to this country a few years back by Renault of France; it was made of a conventional alloy copper believed to contain, nominally, 0.6-0.8% chromium and 0.15% zirconium. Its main body portion was cylindrical, about 15.9 mm (5/8") diameter, with a female taper at the rear end to fit onto an adapter and handle electricity and water in a customary manner. A cylindrical nose projected about 8 mm outwardly from the front end; it was coaxial with the main part of the body. This nose had three or more horizontal circular grooves cut into it to give the impression of a series of projecting annular rings normal to the axis of and being disposed one after the other along the length of the nose. The outer diameter of such ring was almost 8.25 mm and the depth of each groove about 0.875 mm. This, ostensibly, was to permit the expected mushrooming that ensued to break off from time to time, thereby periodically tending to reform the welding face or tip into a diameter that more nearly approximated the original. This cap seems to have found few takers here, if any, to the knowledge of applicants, and it has become somewhat of a curiosity. U.S. Pat. No. 4,476,372 of Oct. 9, 1984, appears to be virtually a copy of such "Renault Cap".
Another earlier-proposed internally-cooled welding cap having somewhat of a nose is described as being made from copper, tungsten, molybdenum, or their alloys, or some precious metal alloys. It is depicted in FIG. 1 of U.S. Pat. No. 3,909,581. Such "copper alloy" tip was held in a surrounding retainer of less expensive metal, which retainer fitted to an electrode shank or adapter; the less expensive metal claimed was aluminum, iron, zinc, and their alloys. An object was to save on the more expensive tip material and have a disposable cap that did not require redressing for economy. Conventional tip shapes at least approximating some commercially popular ones are depicted in FIGS. 2a through 2g of this patent, as is a type of mushrooming in FIG. 5b.
The Briggs U.K. patent No. 1,177,351, applied for in 1966 and published in 1970, first suggested the use of dispersion-strengthened copper in a resistance welding electrode, specifically as the hot end thereof which contacted the workpiece. The remainder (i.e., the main body portion) was to be of copper or a copper alloy. The preferred dispersion-strengthened copper was indicated to have alumina as its disperse refractory phase in a concentration equivalent to 1-4 volume percent of aluminum.
A major step in the use of dispersion-strengthened copper for resistance welding electrode caps occurred about 1975. This is represented by the Shafer and Nadkarni U.S. Pat. No. 4,045,644 of 1977. Here, the dispersion-strengthened copper stock then produced by hot extrusion was formed into electrodes and caps which were cold worked ("upset") to reorientate at least part of the grain structure and thereby improve serviceability.
Further progress in this art is disclosed in the very recent Nadkarni et al U.S. Pat. No. 4,478,787. FIGS. 1 and 2 of that patent depict a cap formed from dispersion-strengthened copper that has a steel jacket swaged around the main part of the body. When such jacket is polished, it glistens; the electrode bears the fanciful trademark Silver Bullet. Such mark, and U.S. Pat. Nos. 4,045,644 and 4,478,787, are owned by the assignee of the instant patent application, SCM Corporation. The teachings of these patents are incorporated by reference in this application.
Additionally, there have been some bimetal welding electrodes patented in recent years wherein the front end is either dispersion-strengthened copper itself or such material sheathed at the tip with a layer of high-conductivity copper. Such articles are adapted for water cooling, and the back end thereof is another metal, typically high-conductivity copper. The front ends shown appear to be fairly blunt, with the broadest diameter equal to or closely approaching the diameter of the main body portion. U.S. Pat. Nos. 4,071,947; 4,288,024; and 4,345,136, to Russel. A. Nippert, are representative here.
Whether the caps for resistance welding in the production of automobiles are of the "copper alloy" composition type or whether they (or at least their tip ends) have been made from dispersion-strengthened copper, the shapes of such cap tips has quite consistently conformed essentially to one or more of the conventional six shapes described above. None of the foregoing "copper alloy" types, of either a "conforming" or a "nonconforming" configuration, have any reputation for being even fairly durable for use in the resistance welding of galvanized steel, and particularly of double-sided galvanized sheet. The caps made of dispersion-strengthened copper showed some modestly improved life in such rigorous service, and this will be demonstrated by actual exemplification to follow.